Can someone read about another religion just to get knowledge?

It depends who that person is and how much settled he is in the faith of the Primal Vow. A person who really has faith in Amida Buddha can read whatever he wants because he will never be influenced by other religions, but someone not yet settled in faith should better leave aside any nonBuddhist book and focus entirely on listening deeply (reading is also listening) the Jodo Shinshu teaching and asking questions to clear his doubts. Why? Because he can be easily influenced by what he reads and everything he reads can become an obstacle and make him depart from the Dharma and the Primal Vow. I saw many people abandoning the Buddhist Path because they read too many things.
 
Also, I remember that Master Rennyo said something like, before shinjin listen to the Dharma, after shinjin also listen to the Dharma. The listening of Dharma never stops for a true Nembutsu follower.
 
So read whatever you want if you have faith and you just want to get knowledge but be careful to never abandon listening and reading the Dharma. Don’t put the study of other religions first and reading the Dharma second. Make yourself a habit to always read at least a Dharma sentence or a Dharma text every day. If you find time to read about other religions you will certainly find time for the Dharma, too.